This invention relates to an electric power steering assist mechanism, and, more particularly, to an electric power steering assist mechanism having an isolated output shaft.
In a typical electric power steering assist mechanism, a reduction gear set applies steering assist to the output shaft in the column. The output shaft comprises an upper shaft and a lower shaft connected by a torsion bar. The upper shaft connects to the steering wheel of the vehicle, and the lower shaft connects to an intermediate shaft that ultimately connects to the rack and pinion gear of the vehicle. The reduction gear is rigidly attached to the lower shaft and is powered by a motor that applies the steering assist to the output shaft. A sensor measures the angular displacement between the upper and lower shafts when torque is applied and the shafts are axially rotated.
Clearances are minimized in the reduction gear mesh, the rack and pinion gear mesh, and the intermediate shaft joints to minimize vibration. Such is typically accomplished with precision components, generally much greater precision than is typically required in order to maintain the minimal clearance and still allow for the rotation of other such components at low friction levels. Further, such has also been accomplished by the addition of compliance in the intermediate shaft. This compliance is introduced by the addition of a torsional isolator. The torsional isolator allows the intermediate shaft and the rack and pinion to move without having to overcome the inertia and friction associated with the motor and the reduction gear set. This significantly reduces the torque across the reduction gear mesh, the rack and pinion gear mesh, and the intermediate shaft joints.
One particular use of a torsional isolator is disclosed in U.S. Pat. No. 5,878,832 entitled xe2x80x9cSteering Apparatus for Motor Vehiclexe2x80x9d, which includes a torsional isolating means comprising a plurality of vibration isolators mounted between a speed reducing gear set and a motor. In this design, only the inertia of the motor is isolated, and the inertia of the gear set remains.
Another use of a torsional isolator involves positioning similar vibrational isolators axially on opposing ends of the worm gear. In this design, still only the inertia of the motor is isolated. Moreover, since the worm gear slides within the bearings located on both ends of the worm gear, additional vibrational interfaces exist.
A design is needed in which the compliance is applied as far downstream in the assist circuit as possible so as to isolate as many vibration points with the compliance means as possible. At the same time, the stiffness between the as driver and the wheels resulting from the compliance means must be minimized. Such a design would require the isolation of both the motor and the gear set. The optimum location for implementing such a design is between the output gear and the output shaft.
An electric power steering assist mechanism for a motor vehicle comprises a speed reducing gear, an output shaft rotatably supported within the speed reducing gear, an isolator disposed between the speed reducing gear and the output shaft, and an electric motor connected to the speed reducing gear to assist in driving the speed reducing gear. An upper end of the output shaft has a steering hand wheel attached to it, and a lower end of the output shaft attaches to an intermediate shaft that connects to rack and pinion gears that ultimately steer the wheels of the motor vehicle. An isolator is positioned between the speed reducing gear and the output shaft to separate the speed reducing gear and the output shaft and to add compliance to the steering assist mechanism. A worm gear is typically used to transfer torque from a driveshaft of an electric motor to the speed reducing gear thereby driving the speed reducing gear.
The speed reducing gear is generally tubular in structure and contains a plurality of ridges that project perpendicularly from the inner surface of the tubular structure and extend longitudinally along the tubular surface. The outer surface of the output shaft likewise contains a plurality of ridges that project perpendicularly from the outer surface and extend longitudinally along the length of the output shaft. The isolator, typically of an elastomeric chemical structure, is disposed in the annular space between the speed reducing gear and the output shaft and provides the requisite compliance to the steering system to minimize vibration.